1. Field of the Invention
This invention relates generally to a magnetic field sensor and more particularly to a magnetic field sensor that makes use of the Lorentz force.
2. Description of the Related Art
Fiber-optic interferometric sensors have become an established technology for Naval underwater acoustic applications. Both platform mounted systems and seabed mounted systems have been developed. See, e.g., Dandridge A., Tveten A. B., Kirkendall C. K. 2004 Development of the fiber optic wide aperture array: From initial development to production, NRL Review (available at www.nrl.navy.mil); Davis A. R., Kirkendall C. K., Dandridge A., Kersey A. D. 1997 64 channel all optical deployable acoustic array Proc. of the 12th Int. Conf. Optical Fiber Sensors, Washington D.C., 616-619; and Cranch G A, Crickmore R, Kirkendall C K, Bautista A, Daley K, Motley S, Salzano J, Latchem J and Nash P J 2004 Acoustic performance of a large-aperture, seabed, fiber-optic hydrophone array Journal of the Acoustical Society of America 115 2848-58, each publication incorporated herein by reference. Fiber-optic sensor based seabed mounted hydrophone arrays offer the potential for very large area coverage with a lightweight, rapidly deployable system. The optical fiber link is capable of carrying information from a large number of fiber-optic sensors (several hundred), to a remotely located shore station or surface mooring. Passive target detection by acoustic signature measurement forms the basis of many sonar systems. However, detection of targets by other associated signatures such as electric or magnetic field is also possible. See, e.g., Bucholtz F, Dagenais D M, Villarruel C A, Kirkendall C K, McVicker J A, Davis A R, Patrick S S, Koo K P, Wang G, Valo H, Eidem E J, Andersen A, Lund T, Gjessing R and Knudsen T 1995 Demonstration of a fiber optic array of 3-axis magnetometers for undersea application IEEE Transactions on Magnetics 31 3194-6, incorporated herein by reference. This can be particularly advantageous in areas of high acoustic reverberation and noise where acoustic detection ranges are limited. An underwater array consisting of a combination of sensors may therefore, in certain circumstances, provide an improved detection capability.
Fiber-optic sensor based magnetometers have many favorable attributes for applications requiring multi-point, remote measurements of low frequency magnetic fields. An undersea magnetometer requiring no electrical power is highly desirable to improve reliability and to enable remote location of the sensors. The sensors are connected by a fiber-optic link free from electromagnetic interference and do not radiate any electric or magnetic fields of their own. A fiber-optic magnetometer has been previously demonstrated that uses a magnetostrictive material to convert the magnetic field into a strain, which is measured interferometrically. See, e.g., Bucholtz F, Villarruel C A, Davis A R, Kirkendall C K, Dagenais D M, McVicker J A, Patrick S S, Koo K P, Wang G, Valo H, Lund T, Andersen A G, Gjessing R, Eidem E J and Knudsen T 1995 Multichannel fiberoptic magnetometer system for undersea measurements Journal of Lightwave Technology 13 1385-95, incorporated herein by reference. The magnetostrictive material was a transversely annealed Metglass cylinder around which optical fiber is wound. This is placed in one arm of an interferometer which measures the strain generated in the presence of a magnetic field. The response of the Metglass to magnetic field is quadratic, such that by applying an AC magnetic dither field to the transducer (typically up to 20 kHz) the low frequency magnetic field of interest appears as modulation sidebands on a carrier at the dither frequency. Low frequency magnetic field resolutions of 3 pT/Hz1/2 at 10 Hz and 38 pT/Hz1/2 at 0.1 Hz have been demonstrated, which compares very well to high performance flux-gate magnetometers achieving low frequency resolutions around 1-10 pT/Hz1/2. See, e.g., Dagenais D M, Bucholtz F, Koo K P and Dandridge A 1988 Demonstration of 3 pt-square-root-(hz) at 10 hz in a fibre-optic magnetometer Electronics Letters 24 1422-3; Dagenais D M, Bucholtz F, Koo K P and Dandridge A 1989 Detection of low-frequency magnetic signals in a magnetostrictive fiber-optic sensor with suppressed residual signal Journal of Lightwave Technology 7 881-7; and Billingsley Magnetics, www.magnetometer.com, all publications incorporated herein by reference. However, magnetostrictive Metglass provides a far from ideal strain response. It has been observed that these materials can exhibit both a significant residual signal in the absence of a magnetic field, which can be equivalent to several μTesla as well as 1/f sideband noise associated with dynamic processes in the metglass. See, e.g., Dagenais D M and Bucholtz F 1994 Measurement and origin of magnetostrictive noise limitation in magnetic fiberoptic sensors Optics Letters 19 1699-701, incorporated herein by reference. Although methods based on choice of dither frequency and annealing conditions have been found to reduce these effects it is generally necessary to operate the sensor closed-loop, maintaining the magnetostrictive at its zero internal field point, to overcome hysteresis and the residual signal in the magnetostrictive. See, e.g., Kersey A D., Jackson D A., Corke M 1985 Single-mode fibre-optic magnetometer with DC bias field stabilization J. Lightw. Technol. LT-3 (4) 836-840, incorporated herein by reference. The fiber-optic interferometer must also be quadrature locked in order to achieve sub-μradian phase resolution. For a three axis magnetometer a total of four feedback loops are required resulting in a relatively complex sensor head when the associated electronics for the feedback loops are included. An array of eight three-axis magnetometers demonstrated magnetic field resolutions of 0.2 nT/Hz1/2 at 0.1 Hz limited by residual 1/f noise. See, e.g., Bucholtz F, Villarruel C A, Davis A R, Kirkendall C K, Dagenais D M, McVicker J A, Patrick S S, Koo K P, Wang G, Valo H, Lund T, Andersen A G, Gjessing R, Eidem E J and Knudsen T. 1995 Multichannel fiberoptic magnetometer system for undersea measurements Journal of Lightwave Technology 13 1385-95, incorporated herein by reference. Although laboratory-based sensors have demonstrated significantly improved performance, consistent improvement in sensitivity has not yet been achieved. The need to provide electrical power and feedback signals to the sensor head is a significant disadvantage, particularly when the sensors are to be located several kilometers from the interrogation system.
An alternative transduction mechanism for a fiber-optic magnetometer has also been demonstrated previously, based on the Lorentzian force generated in a current carrying conductor in the presence of a magnetic field. See, e.g. Okamura H, 1990 Fiberoptic magnetic sensor utilizing the Lorentzian force J. Lightw. Technol. 8 (10), 1558-1564, incorporated herein by reference. A variant of this sensing concept uses a distributed feedback (DFB) fiber laser strain sensor to measure the strain induced in a vibrating metal beam carrying an AC dither current in the presence of a quasi-DC magnetic field. See, e.g. Cranch G A, Flockhart G M H, Kirkendall C K, 2006 DFB fiber laser magnetic field sensor based on the Lorentz force Proc. 18th Int. Conf. Opt. Fib. Sensors, OSA Tech. Digest, ISBN 1-55752-817-9, Cancun, Mexico and Cranch G A, Flockhart G M H, Kirkendall C K, 2008 Optically powered DFB fiber laser magnetometer, SPIE 7004, paper 7004-44, incorporated herein by reference. The DFB fiber laser strain sensor provides an order of magnitude increase in strain resolution compared with the remotely interrogated fiber-optic interferometer, for very short lengths of fiber. See, e.g., Cranch G A, Flockhart G M H, Kirkendall C K, 2007 Comparative Analysis of the DFB Fiber Laser and Fiber-Optic Interferometric Strain Sensors 3rd European Workshop on Optical Fiber Sensors, Naples, Italy, 4-6 July, SPIE 6619, paper 66192C, incorporated herein by reference. This makes it ideally suited for this transduction mechanism where the interaction length is typically a few centimeters. Bending of the beam induces a flexural strain in the fiber in proportion to the Lorentzian force acting on the beam. This force is proportional to the product of the magnetic field strength and current, yielding an AC strain proportional in amplitude to the magnetic field. This transduction mechanism should yield no residual signal for zero applied field and is shown to exhibit no measurable hysteresis. Thus, it should be possible to achieve a stable, drift free magnetic field measurement with a sensor operating open loop. An added benefit is that the responsivity of the sensor is proportional to the current, thus an increase in current will yield a proportional increase in responsivity and sensitivity. The strain induced in the fiber laser modulates the laser emission frequency, which can be converted into an intensity modulation with an imbalanced fiber-optic interferometer located with the interrogation electronics. Thus, no feedback signal is required at the sensor head. The required dither current can be supplied optically removing the need to transmit electrical power to the sensor head.